In reply to Sharp:
With me it was shoulders rather than elbows, but I have certainly found it to be true. I think for two reasons:
(1) everything can rotate freely into a biomechanically natural position, which you can also vary, instead of being fixed in one plane as with a bar.
(2) with rings, if you can be bothered to make the effort - and have them rigged so you can easily adjust the height - you can make sure you do plenty of dips, pressups and rows, not just pullups, and so keep your training more balanced. I always got shoulder trouble doing lots of pullups on a bar, when I switched to doing equal amounts of pullups and dips on rings my shoulders liked it far better